Caption: Unsaturated silica gel spheres. The readiness with which these spheres absorb moisture from the air has led to their use as an industrial drying agent or dessicator. The gel itself is colourless but a blue cobalt salt is added to the spheres which turns pink as they absorb water. When all the spheres are pink, indicating saturation, the gel can be regenerated by heating it up. Silica, silicon (IV) oxide, is a colourless vitreous solid. The rigid gel form is made by coagulating a solution of sodium silicate and heating to drive off water.